This application claims priority from the United Kingdom patent Application No. 9828584.4, filed on Dec. 23, 1998, which is hereby incorporated by reference.
This invention relates to optical filters in general, and more particularly to fiber optic grating temperature compensation devices and methods for compensating for changes in center wavelengths of light passing through fiber optic gratings due to temperature changes.
Optical fibers are important components in modern communication systems. Optical fibers provide for the transmission of optical signals, which are capable of containing massive amounts of information. There has been a need to reflect particular light wavelengths for various applications. Optical filters have been developed for that purpose. A well known, and commonly employed filter is a Bragg filter or Bragg grating.
Optical fiber gratings are often subject to varying environmental conditions, including changes in temperature. The center wavelength of optical fiber gratings is sensitive to changes in environmental temperature.
For the purpose of optical communication systems, the center wavelength of optical fiber gratings should be stable. As optical channel space becomes narrower for higher capacity communication systems, the requirements have become increasingly stringent for controlling and stabilizing the center wavelength of gratings.
The magnitude of the center wavelength variation due to the nonlinearity within common operation temperature range of gratings, xe2x88x9220 to 70 degrees Celsius, can be as high as 50 pm, which is nearly the limit of the allowed variation of the center wavelength of gratings for 100 GHz dense wavelength division multiplexing (DWDM) systems.
U.S. Pat. No. 5,042,898, issued to W. W. Morey and W. L. Glomb on Aug. 27, 1991, proposed the principle of compensating the temperature related shifting of the center wavelength of a fiber optic grating by generating strain in a grating to change the center wavelength in an opposite direction of the shifting. The Morey and Glomb device as well as other conventional existing athermal temperature compensation designs for gratings are all based on the principle that the wavelength variance has a substantially linear relationship with the changes in strain or temperature subjected to the grating. However, this relationship actually has linear and nonlinear portions. These previous conventional devices compensate only the linear portion of the temperature sensitivity of the gratings.
It is further desirable to post-modigy or post tune the amount a compensation device actually compensates for center wavelength variances due to temperature changes. It is also desirable to post-modify or post tune the center wavelength of the grating.
Therefore, a new device and method is required for providing nonlinear compensation of the temperature sensitivity of the center wavelength of fiber optic gratings and for avoiding the disadvantages of the prior art.
It is an object of the present invention to realize nonlinear compensation of the temperature sensitivity of the center wavelength of fiber optic gratings.
A further object of the present invention is to provide a device and a method for post tuning of the compensation of the temperature sensitivity of the center wavelength of fiber optic gratings.
Yet, another object of this invention is to provide a device and a method for post tuning of the center wavelength of fiber optic gratings.
Still, a further object of this invention is to provide a device and a method for nonlinear compensation of the temperature sensitivity of the center wavelength of fiber Bragg gratings, comprising a bi-material structure that generates nonlinear displacement from linear deformation of its components, and a fiber grating mounted on the bi-material structure where nonlinear displacement is generated.
A further object of this invention is to provide a device and a method for post-tuning of the compensation of the temperature sensitivity of the center wavelength of fiber Bragg gratings, including modifying the thermal property of one of the components of a bi-material structure.
Yet, a further object of the present invention is to provide a device and a method for post-tuning of a center wavelength of fiber Bragg gratings, including modifying the dimension of one of the components of the a bi-material structure.
In summary, the present invention discloses a novel fiber optic temperature compensating device comprising a holder, a compensator and a layer. The holder has a first thermal expansion coefficient, a first leg, a second leg and a bar member. The bar member connects the first leg to the second leg and the bar member defines a first portion and a second portion of the device. The holder is operably configured for holding an optical fiber grating between the first leg and the leg in the first portion, whereby the optical fiber grating is operably adapted for passage of light therethrough and for reflecting the light within a range about a center wavelength that varies with changes in ambient temperature of the device and with changes in an axial strain subjected to the optical fiber grating. The compensator has a second thermal expansion coefficient and an end. The compensator is disposed between the first leg and the second leg in the second portion. The second thermal expansion coefficient is greater than the first thermal expansion coefficient so that an increase in ambient temperature of the device causes the compensator to expand at a greater rate than the holder and the compensator applies to the holder a force, located at an applied distance from the bar member, whereby the axial strain of the optical fiber grating is reduced to vary the center wavelength of the grating, providing a compensation for variations in the center wavelength introduced by the increase in ambient temperature. The layer is disposed between the end of the compensator and the first leg of the holder. The layer is flexible so that an increase in ambient temperature will vary the applied distance of the force, making the compensation a nonlinear function of the temperature, whereby an increase in temperature imparts a decrease in the applied distance.
The present invention also discloses a novel device for compensating for changes in a center wavelength of a fiber optic grating due to temperature variations, comprising a holder, a compensator and a layer. The holder has a first leg, a second leg and a bar member connecting the first and second legs. The fiber optic grating is attached to the first and second legs. The compensator is attached to the first and second legs of the holder. The layer is disposed between the compensator and the first leg of the holder. The bar member is disposed between the compensator and the fiber optic grating. The holder and the compensator have respective thermal expansion coefficients. The thermal expansion coefficient of the compensator is greater in value than the thermal expansion coefficient of the holder.
The present invention also discloses a novel device comprising a holder including a first and second connector for securing a fiber optic grating, the first connector is fixed, the second connector is movable to increase or decrease a strain in the grating; and a source of force applied to said second connector, the said source of force varies with changes in temperature, such that with and increase in temperature the second connector moves toward the first connector thereby decreasing the strain in the fiber optic grating.
The present invention also discloses a novel method of compensating for changes of a center wavelength of a fiber optic grating due to temperature variations comprising the steps of: providing a holder having a first thermal expansion coefficient, a first leg, a second leg and a bar member, the bar member connecting the first leg and the second leg and the bar member defining a first portion and a second portion of the device; applying a pre-load axial strain to the fiber optic grating; attaching the fiber optic grating with the pre-load axial strain to the first leg and the second leg in the first portion, whereby the optical fiber grating is operably adapted for passage of light therethrough and for reflecting the light within a range about a center wavelength that varies with changes in temperature of the device and with changes in an axial strain subjected to the optical fiber grating; providing a compensator having an end and a second thermal expansion coefficient that is greater in value than the first thermal expansion coefficient; disposing the compensator between the first leg and the second leg in the second portion, whereby an increase in temperature of the device causes the compensator to expand at a greater rate than the holder and the compensator t, applies to the holder a force, located at an applied distance from the bar member, whereby the axial strain of the optical fiber grating is reduced to vary the central wavelength of the grating, providing a compensation for variations in the central wavelength introduced by the increase in temperature; providing a layer that is flexible; and, disposing the layer between the end of the compensator and the first leg of the holder, so that an increase in temperature will vary the applied distance of the force making the compensation a nonlinear function of the temperature, whereby an increase in temperature imparts a decrease in the applied distance.
These and other objects of the invention will be apparent from the following detailed description.